A switch-mode power supply is a power source which utilizes modern electrical and electronic technology to control the ratio of on-time to off-time for a switch transistor and maintain a stable output voltage. The switch-mode power supply can be categorized into DC-DC converter circuit or AC-DC converter circuit, isolated or non-isolated converter circuit, boost or buck converter circuit, continuous current mode (CCM) or discontinuous current mode (DCM), etc. Among these converter circuits, the isolated converter circuit can further be categorized into single-ended structure (flyback or forward) and double-ended structure (pull-push, half-bridge or full-bridge).
There are about 14 commonly used types of basic topologies for the switch-mode power supply. Each type has its own characteristics and application. The flyback switch-mode power supply is more applicable to a high-voltage and low-power scenario where the voltage is no higher than 5000 V and the power is lower than 15 W. In the case of a relatively high input voltage and an appropriate primary current, the flyback switch-mode power supply may also be applied in a power source whose output power is up to 150 W. A major advantage of the flyback switch-mode power supply is that it does not require an output filtering inductor, which is especially crucial for decreasing the size of the converter and lowering the cost. Moreover, the flyback switch-mode power supply does not require a high-voltage freewheeling diode, either, which is more advantageous in a high-voltage application.
With respect to the topology of the flyback switch-mode power supply, when the switch transistor turns on, the transformer stores energy and the load current is provided by an output filtering capacitor. When the switch transistor turns off, the transformer delivers the stored energy to the load and the output filtering capacitor in order to compensate the energy consumed when the capacitor alone provides current to the load.
FIG. 1 illustrates a basic topology of a flyback switch-mode power supply. The flyback switch-mode power supply 100 includes a flyback converter and a PWM controller 101. The flyback converter includes a transformer 102, a switch transistor S1, an output rectifier D1 and an output filtering capacitor C1. The type of the converter can be recognized easily as the flyback converter through the in-phase terminals of the primary winding L1 and the secondary winding L2 of the transformer.
An input AC voltage VIN is coupled to the in-phase terminal of the primary winding L1 of the transformer. The anti-phase terminal of the primary winding L1 is coupled to the drain of the switch transistor S1. The gate of the switch transistor S1 is coupled to the PWM controller. The source of the switch transistor S1 is grounded. The in-phase terminal of the secondary winding L2 is grounded. The anti-phase terminal of the secondary winding L2 is coupled to the positive terminal of the output rectifier D1. The negative terminal of the output rectifier D1 and the output filtering capacitor C1 are coupled to an output stage VOUT. The other terminal of the output filtering capacitor C1 is grounded.
When the switch transistor S1 turns on, the input AC voltage VIN supplies energy to the transformer via the primary winding L1 of the transformer and the switch transistor S1, the voltage at the anti-phase terminal of the secondary winding L2 is lower than the voltage at the in-phase terminal, the voltage of the output rectifier D1 is inverted and the output filtering capacitor C1 alone provides power to the load. When the switch transistor S1 turns off, the current flowing through the magnetizing inductor of the transformer makes the voltage of the secondary winding L2 inverted and the output rectifier D1 is forwardly biased. The transformer delivers the stored energy to the load and the output filtering capacitor C1. In order to stabilize the output stage VOUT, the PWM controller controls the ratio of on-time to off-time of the switch transistor S1 based on a feedback voltage from the output stage VOUT.
In order to supply power to the PWM controller, the flyback converter may further includes an auxiliary winding L3, a diode D2 and a capacitor C2. The in-phase terminal of the auxiliary winding L3 is grounded. The anti-phase terminal of the auxiliary winding L3 is coupled to the positive terminal of the diode D2. The negative terminal of the diode D2 and the capacitor C2 are coupled to the power supply terminal VCC of the PWM controller chip. The other terminal of the capacitor C2 is grounded.
When the switch transistor S1 turns on, the voltage at the anti-phase terminal of the auxiliary winding L3 is lower than the voltage at the in-phase terminal, the voltage of the diode D2 is inverted and the capacitor C2 alone supplies power to the PWM controller. When the switch transistor S1 turns off, the current flowing through the magnetizing inductor makes the voltage of the auxiliary winding L2 inverted, the diode D2 is forwardly biased, and the auxiliary winding L3 supplies power to both the PWM controller and the capacitor C2.
When the abnormal situation occurs where the output stage of the flyback switch-mode power supply is shorted, the current flowing through the secondary winding L2 increases sharply, which consumes a majority of the energy supplied by the primary side of the transformer, and the output voltage decreases so that feedback voltage decreases and thus the output of the PWM controller reaches a full duty cycle. At the meantime, the voltage at the anti-phase terminal of the auxiliary winding L3 becomes too low to supply power to the PWM controller and the capacitor C2. As a result, the power supply voltage VCC of the PWM controller decreases. When VCC decreases to a preset voltage, the PWM controller turns off, the power supply voltage VCC begins to increase to enter a restart stage thereby realizing the so called Hiccup protection.
FIG. 2 illustrates a waveform of the power supply terminal VCC and the drive terminal GD of the PWM controller chip when the abnormal situation where the output stage of the flyback switch-mode power supply is shorted occurs. However, the time for entering short-circuit protection is determined by the capacitor C2 and the power of the PWM controller. If the capacitance of the capacitor C2 is too large, it will take a long time to enter short-circuit protection and the current will be large when the system is shorted, also, the transformer will be saturated and the power transistor will be burned down thus damaging the circuit. If the capacitance of the capacitor C2 is too small, the time will be shortened, but the PWM controller may not be able to work normally. Therefore, it is difficult to test the performance of the short-circuit protection, and the uniformity of the batch production will be poor.